The archetypal catheter-type system to release a self-expanding stent into a bodily lumen takes the form of a catheter shaft extending between a proximal end of the system that remains outside the body and a distal end of the system that carries the stent. The proximal end is manipulated, to release the stent. This manipulation involves pulling on one component of the system, at the proximal end, in order to pull back proximally a sheath that radially confines the stent.
Since the stent is exerting radially outward force on the sheath, up to the time that the stent is released from the sheath, the stent will tend to be carried with the sheath, whenever the sheath is pulled proximally, relative to the shaft of the catheter system. Thus, that shaft needs to engage with a surface portion of the stent and restrain the stent as the sheath retracts proximally, relative to the stent. This restraining of the stent naturally imposes on the catheter shaft an endwise compressive stress, as that shaft “pushes” on the stent during proximal withdrawal of the sheath.
Thus, the shaft of the catheter system inherently includes components that are sliding over each other, throughout the length of the system, which length can be rather large, typically between 100 cm and 150 cm.
As experience grows, self-expanding stents can be delivered to ever more demanding locations in the body, with the delivery system advancing along ever more tortuous and confined lumens. Today, there is vigorous demand for delivery systems that can keep to an overall diameter of 6 F or smaller (6 F equals 2 mm, with the unit of “1 French” corresponding to ⅓ mm). As the diameter is continually brought down, so the pressure on the system designer increases, to minimise the forces of friction that arise with sliding contact along the shaft length.
Further, with greater radial force, greater axial length, and with ever more sophisticated coverings on the stent strut matrix the “drag” of the stent on the proximally-moving sheath gets bigger. This, in turn, raises the tension required in the stent release means. It can well be imagined that a need therefore exists to hold down the absolute values of endwise tensile stress, to values that the system can tolerate without kinking or buckling and without failure of any one of the very long, very thin components involved.